Fix driver.dart

pkg/analysis_server/lib/driver.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library driver;
 
+import 'dart:async';
 import 'dart:io';
 
 import 'package:analysis_server/http_server.dart';
 import 'package:analysis_server/src/socket_server.dart';
 import 'package:analysis_server/stdio_server.dart';
 import 'package:analyzer/src/generated/java_io.dart';
 import 'package:analyzer/src/generated/sdk.dart';
 import 'package:analyzer/src/generated/sdk_io.dart';
 import 'package:args/args.dart';
 
@@ skipping 149 matching lines @@
   /**
    * Print information about how to use the server.
    */
   void _printUsage(ArgParser parser) {
     print('Usage: $BINARY_NAME [flags]');
     print('');
     print('Supported flags are:');
     print(parser.usage);
   }
 }